The present invention relates to a control device of a compression-ignition engine.
For example, JP2009-197740A discloses an engine which performs compression-ignition combustion of mixture gas inside a cylinder when an operating state of the engine is within a predetermined range on a low engine speed side and on a low engine load side. In the engine, for performing the compression-ignition combustion, a close timing of an exhaust valve is set to before a (compression) top dead center and an open timing of an intake valve is set to after the top dead center to provide a period in which the exhaust and intake valves are both closed over the top dead center (i.e., negative overlap period). By providing the negative overlap period, a part of high-temperature exhaust gas remains within the cylinder. In other words, a temperature inside the cylinder is increased by an internal EGR (Exhaust Gas Recirculation), and thus, ignitability in the compression ignition and combustion stability improve. JP2009-197740A also discloses that within an operating range of the engine where the compression-ignition combustion is performed, an EGR ratio by the internal EGR is increased by advancing the close timing of the exhaust valve more as the engine load becomes lower.
Moreover, JP2007-120330A discloses an engine for a gas heat pump in which, similarly to JP2009-197740A, the EGR ratio by the internal EGR is increased when performing the compression-ignition combustion, by extending the negative overlap period longer as the engine load becomes lower.
Generally, as the engine load becomes lower, a temperature environment inside a cylinder degrades, and thus, it is disadvantageous when performing compression-ignition combustion. Therefore, increasing the EGR ratio by the internal EGR more as the engine load becomes lower as disclosed in JP2009-197740A and JP2007-120330A increases the temperature inside the cylinder when the piston reaches the compression top dead center (i.e., a compression-end temperature) since an amount of the exhaust gas introduced in to the cylinder is increased and the temperature inside the cylinder before compression stroke starts is increased. Thus, the increase in the EGR ratio described above can be advantageous in view of the ignitability in the compression ignition and the combustion stability.
However, through the study by the present inventors, it was discovered that setting the EGR ratio higher than a predetermined ratio would result in decreasing the compression-end temperature. This degrades the ignitability in the compression ignition and the combustion stability when the engine operating state is within a range where the engine load is low or extremely low.